After extensive use blowout preventers may not quite seal when they close. Such failure to seal may result from a damage to the packing face, from imperfections in the pipe extending through the blowout preventer or from the inclusion of trash. When such failure to seal occurs, it is desirable to have a way to seal obtained without retracting the rams and thereby losing control of the well. In some installations the preventers may be hundreds of feet below the water surface.